The use of composite materials in the construction of military and commercial aerospace vehicles is widespread and increasing. The most commonly used composites consist of a polymer matrix reinforced with fibers of such materials as carbon, glass and Kevlar.
In many cases relatively thin composite panels are used. At the present time, panels 0.088 to 0.189 inches in thickness appear to be most common, although thicker and thinner parts are also used.
To fasten such panels together, adhesive bonding is used wherever possible. When adhesive bonding alone is judged insufficient, fasteners are used in addition to, or in place of, adhesives.
In the manufacture of conventional aluminum aircraft, solid aluminum rivets have been used successfully in very large quantities to fasten the sheet thicknesses mentioned, starting in about 1935 and continuing to the present time. Aluminum rivets are not suitable, however, for use in composite sheet material for several reasons. Some composite materials cause accelerated corrosion of aluminum rivets. The coefficients of expansion of aluminum and the composite materials may be too widely different. A lightning strike problem can arise when aluminum or other metal rivets are used in a composite material. For these reasons a need exists for a rivet which is itself made from composite material.
Considerable work is now being done to enable rivets made from composite material to be used to fasten composite sheets, with or without prior adhesive bonding. Up to the present time, these composite solid rivets have been fabricated using the injection molding process to form a rivet having the required manufactured head shape at one end of the usual cylindrical shank portion. The shank portion is made sufficiently long to pass through the workpieces to be fastened and then protrude approximately two shank diameters beyond. Later, this two diameter protruding length is upset to form a "shop formed" head, which in conjunction with the manufactured head serves to hold the workpieces together.
Examples of suitable materials for making composite solid rivets by injection molding are PEI (polyetherimide) reinforced with short glass fibers and PEEK (polyetheretherketone) reinforced with short carbon fibers. Both these materials belong to a composite materials category which is commonly known as "thermoplastic." Such materials become soft and formable at temperatures in the 600.degree. to 700.degree. range but when cooled to room temperature exhibit useful structural strength.
Rivets made from these short fiber reinforced thermoplastic materials have typical average shear strengths of about 16 KSI when measured using the method of MIL-STD-1312 Test 20 and have typical average tension strength in the range 14 to 16 KSI. These rivets may be upset with any simple tooling which is capable of providing a heat input to the two diameters of protruding rivet tail, followed by a pressure to upset this tail when it becomes soft and formable.
This riveting process seems to be gaining favor for fastening composite materials because the rivets themselves are easily made on readily available, high production injection molding machines and they are inexpensive compared to other suitable fasteners for composites, such as titanium threaded shear pins and collars. Also, experts in the composite aircraft field believe that the drilling of holes in a workpiece and the insertion and upsetting of these rivets can be easily automated. In this way, fastening may be accomplished with a simple inexpensive fastener using an inexpensive and reliable installation method.
In some thick or high bearing strength thin workpieces the 16 KSI shear strength and the 12 to 14 KSI tension strength of common short fiber reinforced thermoplastic materials is not adequate. Thus the need exists for a rivet made from composite material and having higher shear and tension strengths.
Long fiber reinforced threaded shear pin type fasteners having 40 to 60 KSI average ultimate shear strength have been described in U.S. patent application Ser. No. 397,659. High shear strength rivets can also be made from laid up panels as described in that application or from pultruded rod. However, it is difficult to upset rivets made in this way in a workpiece. Accordingly, there exists a need for a simple and practical method of upsetting rivets made from long fiber reinforced material. There also exists a need to provide a shop upset profile which is sufficiently strong in tension to optimize lap joint shear strength.